The physiological factors which regulate parturition and the placental production of hormones in primates are poorly understood. The rate of placental blood flow is considered an important factor in influencing the production of placental hormones, but there is virtually no direct evidence to support this claim. Our primary goal is to clarify the physiological relationship among endocrine and vascular events in the initiation of parturition in the rhesus monkey. Major emphasis will be placed on physiological studies in unanesthetized chronic fetal preparation in pregnant rhesus monkeys of a) the spontaneous hormonal and circulatory changes in the mother and fetus which precede parturition and which occur during labor; b) the influence of graded reductions in uteroplacental blood flow on the production of estradiol, estrone, progesterone, and monkey chorionic somatomammotropin (mCs) by the fetus and placenta; c) the manner in which prostaglandin synthetase inhibitors (e.g., indomethacin) influence myometrial contractility and uteroplacental blood flow in primates. Measurements of blood flow will be made by electromagnetic flowmeter techniques and radioactive microspheres, and plasma hormones will be measured by specific radioimmunoassays.